(1) Field of the Invention
This invention relates to a magnetic recording medium such as a magnetic tape. More particularly, it relates to a magnetic recording medium having improved magnetic characteristics and durability.
(2) Description of the Related Art
A magnetic recording medium such as a magnetic tape or a magnetic card is generally made by coating a magnetic coating solution comprising a magnetic powder and a binder as a magnetic layer on a substrate such as a polyester film. Recently, to improve the S/N ratio and the recording density by increasing the coercive force and maximum saturation magnetization, a finely divided magnetic powder having a large specific surface area has been used as the magnetic powder, and either iron oxide magnetic powders or metallic iron magnetic powders having a specific surface area such as 60 m.sup.2 /g, a major axis such as 0.1 .mu.m and a minor axis such as 0.01 .mu.m are used.
When a magnetic coating solution is prepared by using a magnetic powder as mentioned above together with a hydroxyl group-containing resin binder generally used for magnetic layers, such as nitrocellulose or a vinyl chloride/vinyl acetate/vinyl alcohol copolymer, the amount of the magnetic powder adsorbed in the resin binder is increase to thereby increase the viscosity, with the result that the dispersion of the magnetic powder becomes poor. Accordingly, when this coating solution is coated on a non-magnetic support to form a magnetic layer, the orientation of the magnetic powder is poor and the dispersion of the magnetic powder in the magnetic layer is unsatisfactory, and the magnetic characteristics and electro-magnetic conversion characteristics of the magnetic recording medium become poor. This tendency is conspicuous as the size of the magnetic powder is made finer to improve the recording density, and the specific surface area is increased, for example, to 30 m.sup.2 /g or more.
As the means for improving the dispersibility of the magnetic powder in a magnetic coating solution, there has been adopted a method in which various surface active agents or fatty acids are incorporated as the dispersant in magnetic coating solutions, or a method in which the magnetic powder is surface-treated with surface active agents or fatty acids (see, for example, Japanese Unexamined Patent Publication No. 58-102504). Furthermore, there has been proposed a method in which the magnetic powder is surface-treated with a silane coupling agent or a titanium coupling agent (see, for example, Japanese Unexamined Patent Publication No. 55-125539 and Japanese Unexamined Patent Publication No. 56-58135).
According to the method in which a dispersant is added into a magnetic coating solution, however, to obtain a required dispersing effect, a large amount of the dispersant must be used, and therefore, problems such as a reduction of the coating strength of the magnetic layer and a bleeding of the dispersant to the surface of the magnetic layer, causing a reduction of the commercial value, arise.
Moreover, according to the method in which the magnetic powder is surface-treated with a fatty acid or a surface active agent, since the surfaces of particles of the magnetic powder are made hydrophobic, the orientation is greatly improved, but since the amount of the magnetic powder adsorbed in the binder is too small, the stability of the coating solution is lowered and the strength of the formed magnetic layer is reduced or the moisture resistance becomes poor, and a reduction of the running property under high-temperature and high-humidity conditions or the running durability under usual temperature and humidity conditions often occurs.
According to the method in which the magnetic powder is surface-treated with a coupling agent, an improvement of the durability of the magnetic layer due to the improved wettability between the magnetic powder and the binder is expected. In practice, however, the reaction between the coupling agent and the magnetic powder is poor or the dispersibility of the magnetic powder becomes unsatisfactory, and a reduction of the electro-magnetic conversion characteristics often occurs.
Furthermore, to prevent an irregular winding and improve the antistatic and running properties, there is widely adopted a method in which a backcoat layer composed of a non-magnetic powder, such as electroconductive carbon black, incorporated into a binder is formed on the surface of the non-magnetic support where the magnetic layer is not formed. Especially, for a vacuum evaporation tape where a magnetic layer is formed by directly depositing a ferromagnetic metal on the non-magnetic support by vacuum evaporation or the like, this backcoat layer is indispensable.
In this backcoat layer, the non-magnetic powder-dispersing property of the binder and the properties of the formed coating are important factors having an influence on the durability and running property. For example, if a binder customarily widely used, for example, a vinyl chloride/vinyl acetate/vinyl alcohol copolymer, nitrocellulose, a polyurethane or a polyester is directly used, the dispersibility is poor, and therefore, the coating layer is scratched by a pinch roller or a guide pin and the powder falls off after many runs.
A method in which a surface active agent is used as the dispersant for the backcoat layer is considered effective as a means for further improving the durability or running property, but since the surface active agent is present in the backcoat layer, there is a risk that problems concerning the mechanical strength and durability will arise, for example, a falling off of the powder and blooming with a lapse of time.